This invention relates to the survey of the peripheral field of vision of the human eye utilizing an instrument known as a visual field tester.
A visual field tester surveys the sensitivity of a patient's retina. In what is called a static threshold test, a spot of light, termed the stimulus, is projected to a hemispherical projection screen for a short period of time. A patient viewing the hemispherical projection screen from the center of the sphere fixates along a line of sight to a fixation light source mounted on the surface of the bowl. The point of projection on the hemispherical projection screen is changed in a controlled fashion from one point to another in a predetermined pattern of points spaced apart from the fixation point. The intensity of light at a point varies from presentation to presentation in a controlled fashion. Preferably, the point is varied in intensity as the point moves from position to position on the hemispherical projection screen. A subjective determination is made by the patient in depressing a response button, if the stimulus is seen. By positioning the point to known locations on the hemispherical projection screen and changing the brightness (in a total amount of about four decades), the sensitivity of the patient's retina is measured and mapped.
It is known to provide occlusion of the non-tested eye in "white light" field testing of the human eye. This is necessary to ensure that the patient is only responding to light entering the tested eye. Specifically, and from about 1984, it has been known to conduct field tests by occluding the non-tested eye with a white light translucent patch. This white light translucent patch furnishes the non-tested eye with a background illumination similar to that background illumination seen by the tested eye but destroys contrast to the extent that the stimulus cannot be detected. This technique has been used with the reasoning that the provision of a white light background to the untested eye prevents retinal rivalry from obscuring the results of the test.
Retinal rivalry is a phenomenon in some patients where one eye, which is not the subject of test, interferes with the vision of the eye being tested. Retinal rivalry most frequently occurs when the eye not being tested is completely obscured so that the patient sees nothing--and the eye transmits a "black field" to the brain. Specifically, and while the patient being tested is trying to see and participate in a vision test of one eye, this eye is perceived by the patient to become either periodically dark or completely dark. The patient sees nothing. This phenomenon is particularly pronounced where a patient has a so-called "dominant eye" where one eye undertakes the transmission of most of the patient's visual information to the brain.
In attempts to avoid this phenomenon in white light tests, so-called translucent occluders have been utilized. Unfortunately, such translucent occluders utilized in white light field tests have not been completely satisfactory, since the white light field test includes presenting a relatively brilliant stimulus to the patient. Taking the case where the field of vision of the tested eye is either at or below threshold at a tested point, it is possible for the patient being tested to respond using the untested eye to the relatively bright light source passing through the diffuse eye occluder. The person conducting the examination does not know whether the response is due to light being seen by the tested eye or to a "flash" being seen at the non-tested eye through the translucent white light occluder.
Recently, field tests have been expanded to include non white light field testing. An example of this is field tests conducted with a yellow background light with a relative bright point being presented in blue. Such tests are believed more effective in locating early symptoms of eye disease, such as glaucoma.